Understanding the regulatory mechanisms that underlie gene expression during the specification of the myeloid cell lineage is still limited. The inducible expression of the myeloid-specific beta2 integrin CD11b gene in response to physiologic differentiation signals is directed by a compact proximal promoter, that lacks a TATA-box (as do other beta2 integrin promoters), but contains a GC-rich region. Promoters with a similar architecture were thought to control constitutive gene expression. In studies conducted during the previous funding cycle, we found that expression of CD11b in myeloid cells is regulated not only by the primary nucleotide sequence of its promoter, but also by the secondary structure these nucleotides assume. A single-stranded transcriptional activator and a promoter in sequence- and conformation-specific manner, and regulate expression of the CD11b gene in differentiation myeloid cells. Binding sites for these tow factors are present or predicted in the proximal promoters of several genes (including other integrins that are also expressed in myelomonocytic cells. The cloned repressor belongs to the Krupple-type zinc finger family. In contrast t the ubiquitous expression of the activator and the repressor in other cell types, they are lacking (functionally and/or physically) in myeloid cells unless these cells are induced to differentiate. In this proposal, we plan to explore the mechanisms for the local and global alterations in DNA conformation and topology in the CD11b promoter, clone the single-stranded DNA binding activator, define the respective DNA recognition sequences of the two factors, elucidate the mechanisms of repression and activation, and determine the in vivo role of the two factors using myeloid differentiation models and whole mice. These studies should be useful in delineating the molecular basis for the regulated expression of beta2 integrin genes, with implications for similarly organized promoters, and should provide better insight into the factors that regulate gene expression in myeloid cells under normal and pathologic states.